The present invention relates generally to the field of electronic data storage and retrieval systems. In particular, the present invention relates to a magnetoresistive head assembly having asymmetrical circuitry for electrostatic discharge and electrical overstress protection.
In an electronic data storage and retrieval system, a magnetic head typically includes a reader portion having a magnetoresistive (MR) sensor for retrieving magnetically-encoded information stored on a magnetic disc. MR sensors fall generally into two broad categories: (1) anisotropic magnetoresistive (AMR) sensors and (2) giant magnetoresistive (GMR) sensors. AMR sensors generally having a single MR layer formed of a ferromagnetic material. The resistance of the MR layer varies as a function of cos2θ, where θ is the angle formed between the magnetization vector of the MR layer and the direction of the sense current flowing in the MR layer.
GMR sensors have a series of alternating magnetic and nonmagnetic layers. The resistance of GMR sensors varies as a function of the spin-dependent transmission of the conduction electrons between the magnetic layers separated by the nonmagnetic layer and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and nonmagnetic layers and within the magnetic layers.
GMR sensors using two layers of ferromagnetic material separated by a layer of nonmagnetic electrically-conductive material are generally referred to as spin valve (SV) sensors. The layers of a SV sensor include a nonmagnetic spacer layer positioned between a ferromagnetic pinned layer and a ferromagnetic free layer. A magnetization of the pinned layer is fixed in a predetermined direction, typically normal to an air bearing surface (ABS) of the SV sensor, while a magnetization of the free layer rotates freely in response to an external magnetic field. An antiferromagnetic material is typically exchange coupled to the pinned layer to fix the magnetization of the pinned layer in a predetermined direction, although other means of fixing the magnetization of the pinned layer are available.
GMR sensors using two layers of ferromagnetic material separated by a layer of nonmagnetic electrically-insulating material are generally referred to as spin-dependent tunnel junction (STJ) sensors. The layers within a STJ sensor include an ultra-thin tunnel barrier layer positioned between a ferromagnetic pinned layer and a ferromagnetic free layer. As in the SV sensor, a magnetization of the pinned layer is fixed in a predetermined direction, typically normal to an air bearing surface of the STJ sensor, while a magnetization of the free layer rotates freely in response to an external magnetic field. An antiferromagnetic material is typically exchange coupled to the pinned layer to fix the magnetization of the pinned layer in a predetermined direction, although other means of fixing the magnetization of the pinned layer are available.
Such MR sensors are particularly sensitive to electrostatic discharge (ESD) and electrical overstress (EOS) during both manufacture and use of the magnetic head. Generally speaking, ESD is the discharge of electrostatic charges to or from the magnetic head (i.e., an assembler accidentally touches the magnetic head with metal tweezers and causes a spark to the MR head), while EOS is the application of a current or voltage to the head that exceeds its safe operational limits (for example, too much sense current is accidentally provided to the MR sensor during testing).
This sensitivity to electrical damage is particularly severe for MR sensors because of these sensors' relatively small physical size. The discharge of only a few volts through such a physically small resistor is sufficient to produce currents capable of severely damaging or completely destroying the MR sensor. The nature of the damage that may be experienced by an MR sensor varies significantly, including complete destruction of the sensor via melting and evaporation, contamination of an air bearing surface, generation of shorts via electrical breakdown, and milder forms of damage in which head performance may be degraded, such as sensor amplitude loss.
A common solution to the problem of ESD and EOS on magnetic heads is the use of protection circuitry connected to the MR sensor to divert large currents from the MR sensor. This circuitry generally includes nonlinear circuit components, such as diodes, transistors, and varistors, metal-semiconductor-metal and metal-insulator-metal tunnel junctions, and spark gaps configured to divert current from the MR sensor when a voltage across the MR sensor would otherwise exceed a predetermined threshold. Prior art designs assume that the electrical protection needed in both directions through the MR sensor is identical with respect to current polarity, and thus symmetrically design the circuitry for protection against excessive currents in both directions through the MR sensor.
In general, solutions to the problem of ESD and EOS sensitivity require tradeoffs between the degree of protection offered, magnetic head performance, and manufacturing cost. In particular, the addition of such protective circuitry effectively adds parasitic resistance and capacitance across the MR sensor, which adversely affect magnetic head performance.
ESD and EOS protection circuitry need only be electrically connected to the MR sensor. Thus, the ESD and EOS protection circuitry can be physically located anywhere within the electronic data storage and retrieval system. It is generally considered better, however, to locate the protection circuitry as close to the MR sensor as possible. If the protection circuitry is built on a suspension arm that suspends the magnetic head above a magnetic media, it cannot protect the MR sensor from ESD damage caused prior to the mounting of the MR sensor onto the suspension arm. Furthermore, protection strategies using circuits located physically closer to the MR sensor have a better protective response speed and effectiveness than those using circuits located distant from the MR sensor. It has been suggested that the protection circuitry be built directly upon a slider body upon which the magnetic head is built. However, few prior art techniques of building silicon semiconductor devices on traditional alumina-titanium-carbide (AlTiC) slider bodies exist (and such known processes are difficult and/or ineffective to implement), and processing issues still exist with the use of silicon slider bodies.